1. Field of the Invention
The present invention relates to absolute angle detection apparatuses, and more particularly, to an absolute angle detection apparatus that detects an absolute angle of a rotating body using Gray codes.
2. Description of the Related Art
Vehicles having an absolute angle detection apparatus disposed between a steering shaft and a vehicle body are known. In such a vehicle, damping force control of a suspension, shift position control of an automatic transmission, rear-wheel steering control for four-wheel steering vehicles, etc., are performed on the basis of a steering angle, a steering speed, a steering direction, etc., of a steering wheel detected by the absolute angle detection apparatus.
Devices using binary coded decimal (BCD) codes, M-sequence codes, Gray codes, etc., are known as devices for detecting an absolute angle with a certain resolution. In particular, the device using Gray codes is advantageous in increasing the resolution since the Gray codes at the adjacent steps always differ from each other at only one bit and it is not necessary to use a reading timing pulse. Therefore, the device using the Gray codes is most commonly used.
In a typical absolute angle detection apparatus using Gray codes, code pattern lines corresponding to first to third digit lines are formed on a disc, and pickups 1 to 9 that function as detection elements are arranged so as to face the code pattern lines within an angular range of about 90°. Five-bit code elements supplied from the pickups 1 to 5 arranged so as to face the code pattern line corresponding to the first digit line are combined to obtain Gray codes of the first digit line with a resolution of 1.125° and a period of 22.5°. Two-bit code elements supplied from the pickups 6 and 7 arranged so as to face the code pattern line corresponding to the second digit line are combined to obtain Gray codes of the second digit line with a resolution of 22.5° and a period of 90°. Two-bit code elements supplied from the pickups 8 and 9 arranged so as to face the code pattern line corresponding to the third digit line are combined to obtain Gray codes of the third digit line with a resolution of 90° and a period of 360°. An example of such an absolute angle detection apparatus is disclosed in Japanese Unexamined Patent Application Publication No. 2000-28396 (FIGS. 1 to 4).
However, as is clear from FIGS. 2 to 4 of Japanese Unexamined Patent Application Publication No. 2000-28396, when, for example, a rotational angle of a disc 10 changes from 22.5° to 23.625°, the 9-bit code changes from (100010000) to (000010100). Thus, the 9-bit code changes at two bits at the switch position between periods of the Gray codes of each digit line. Therefore, if a signal phase shift occurs due to a physical cause, such as electrical or mechanical influence, at the switch position between the periods of each digit line, the digit lines cannot be identified and the rotational angle cannot be detected accurately.